1. Field of the Invention
The present invention relates to an amplifier circuit and an electronic apparatus including the same, such as a speaker-driving amplifier circuit and an electronic apparatus including the same.
2. Description of the Related Art
FIG. 4 shows a conventional amplifier circuit. Referring to FIG. 4, an amplifier circuit 1 includes an input terminal 2, three amplifiers 3, 4, and 6 which are connected in series from the first stage to the final stage, a signal blocking circuit 5, a detection resistor 7, an output terminal 8, and a comparator circuit 9. In particular, the amplifier 6 is a buffer amplifier that can generate a large output current even when the amplification factor is small. Sometimes the buffer amplifier is attached externally, separate from the other amplifiers. In such a case, it is often impossible to provide a built-in function (feedback loop) wherein the output current is returned in feedback manner to the input side and the output current is prevented from becoming too high.
The input terminal 2 is connected to the signal blocking circuit 5 through the amplifiers 3 and 4 in this order. The signal blocking circuit 5 is connected to the output terminal 8 through the amplifier 6 and the detection resistor 7 in this order. Both ends of the detection resistor 7 are connected to the comparator circuit 9. The output of the comparator circuit 9 is connected to the signal blocking circuit 5. The comparator circuit 9 rectifies two input signals and determines whether or not the difference between the two input signals exceeds a predetermined value.
In the amplifier circuit 1 arranged as described above, the amplifiers 3, 4, and 6 are arranged so that the output voltages sequentially increase starting from the amplifier 4 at the first stage to amplifier 6 at the final stage. On the basis of the potential difference across the detection resistor 7, the comparator circuit 9 detects the magnitude of the output current of the amplifier 6 at the final stage. For example, when the output terminal 8 is short-circuited, causing a high current to flow into the detection resistor 7, the potential difference across the detection resistor 7 increases. The comparator circuit 9 detects when the potential difference exceeds a predetermined level and controls the signal blocking circuit 5 to block a signal output from the amplifier 4 from entering the amplifier 6. As a result, excessive current is prevented from flowing out from the amplifier 6. Accordingly, even when the buffer amplifier is externally attached to the amplifier circuit 1, the amplifier 6 is protected from excessive current caused by short-circuiting of the output terminal.
In the amplifier circuit 1 shown in FIG. 4, the detection resistor 7 is arranged in series in a signal path. Thus, the output impedance of the amplifier 6 increases, and the dumping factor deteriorates. Also, the transient characteristic deteriorates, and the steepness of the rising characteristic is lost. A current that flows through the detection resistor 7 is used to detect excessive current. When the output current value is small, such as when a sound output from an audio system is small, and when the output terminal 8 is short-circuited, the current that flows through the detection resistor 7 is too small to detect excessive current. As a result, the output short current continues to flow.
In order to overcome the problems described above, preferred embodiments of the present invention provide an amplifier circuit that can detect a short circuit even when output current is low and that can protect an amplifier via a simple circuit configuration, and also provide an electronic apparatus including such a novel amplifier circuit.
According to a preferred embodiment of the present invention, an amplifier circuit includes at least two amplifiers connected in series from the first stage to the final stage or one amplifier which functions as an amplifier at the first stage and an amplifier at the final stage. The amplifier circuit includes a detection circuit for detecting a variation in the amplification by comparing the input voltage of one amplifier with the output voltage of the amplifier at the final stage, and an output stopping circuit for stopping the output of the amplifier at the final stage based on the detection result obtained by the detection circuit.
According to another preferred embodiment of the present invention, an amplifier circuit includes at least two amplifiers connected in series from the first stage to the final stage or one amplifier that functions as both as an amplifier at the first stage and an amplifier at the final stage. The amplifier circuit includes a detection circuit which is connected at the input side of one amplifier and at the output side of the amplifier at the final stage and which detects a variation in the amplification by comparing the voltage of the one amplifier with the voltage of the amplifier at the final stage, and an output stopping circuit which is connected to the output of the detection circuit and which stops the output of the amplifier at the final stage based on the detection result obtained by the detection circuit.
It is preferred that no feedback loop from the amplifier at the final stage to the amplifier at a previous stage is included.
The detection circuit may include a comparator that compares the input voltage of one amplifier with the voltage of the amplifier at the final stage and that detects a state in which the ratio between the two voltages changes.
The detection circuit may include a comparator that compares the input voltage of one amplifier with the output voltage of the amplifier at the final stage and that detects a state in which the output voltage of the amplifier at the final stage falls below the input voltage of the one amplifier.
The output stopping circuit may include a power-off circuit that cuts off the power supply to the amplifier at the final stage.
The output stopping circuit may include a signal blocking circuit that blocks an input signal from entering the amplifier at the final stage.
According to another preferred embodiment of the present invention, an electronic apparatus includes an amplifier circuit according to preferred embodiments described above.
According to an amplifier circuit of preferred embodiments of the present invention, it is possible to detect a short-circuited output using a simple circuit configuration and to protect an amplifier at the final stage from damage resulting from a short-circuited output.
According to an electronic apparatus of a preferred embodiment of the present invention, reliability and performance of the apparatus are greatly improved.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.